In recent years, video cameras and electronic cameras using a solid-state image sensor of a CCD (Charge Coupled Device) type, a CMOS (Complementary Metal Oxide Semiconductor) type (also referred to as an amplification type), or the like, have become widely used. In the solid-state image sensor, a plurality of pixel units is arranged in the form of a two-dimensional array, in which the pixel unit includes an optical receiving part that converts receiving light to an electrical signal, signal lines for reading the electrical signal of the optical receiving part, and the like. Light incident from a photogenic subject through a shooting lens of a video camera or electronic camera using a solid-state image sensor is focused on pixels of the sensor arranged in a matrix, converted to an electrical signal by the optical receiving part, and read as an image signal via the signal lines.
A typical solid-state image sensor has a structure in which a photodiode, which is an optical receiving part, is formed on a semiconductor substrate (silicon substrate). A transfer contact for forwarding an electric charge accumulated by the photodiode is formed via an insulation film, and on top thereof is an interlayer insulation film. The sensor further includes a light-shielding film for preventing light from striking any part of the sensor other than the optical receiving part, and a surface protection film. These layers and films are laminated in this order. However, with the solid-state image sensor having such a structure, incident light onto the optical receiving part reflects on a surface of the silicon substrate or a boundary face between the thin films. This reflection loss reduces the light reaching the photodiode, thus resulting in decreased sensitivity of the solid-state image sensor.
In particular, the refractive index of silicon is high, so there are more reflections from silicon and thus the reflection loss will increase. In order to solve this problem, a method for forming an anti-reflection film, such as a silicon nitride film (SiN), on the optical receiving part, thereby reducing the reflection loss of incident light and improving the sensitivity of the solid-state image sensor, has been contemplated. For example, Japan Unexamined Patent Application Publication No. 2000-196051 (referred to herein as “Patent Document 1”) describes a double layer structure of high-refractive-index layer for reducing the reflection loss of incident light near a wavelength of 550 nm. Moreover, Japan Unexamined Patent Application Publication No. 2005-142510 (referred to herein as “Patent Document 2”) describes a method for optimizing the reflection loss by changing the film thickness of each pixel with a single layer of anti-reflection film.
Moreover, the reflectivity characteristic is a critical issue not only in the solid-state image sensor but also in display devices, such as liquid crystal display devices and EL (Electro Luminescent Display) devices. Thus, there is a need for an anti-reflection structure having a low reflectivity characteristic over a wide range of visible light. For example, Japan Unexamined Patent Application Publication No. H10-311903 (Referred to herein as “Patent Document 3”) describes the following technique. A high-refractive-index layer having a refractive index from 1.9 to 2.5 and a low-refractive-index layer having a refractive index from 1.3 to 1.5 are laminated alternatingly on a transparent material to provide an anti-reflection layer as the anti-reflection material of a display device. The high-refractive-index layer and low-refractive-index layer are composed of metallic oxides having different refractive indices. Each of the laminated high-refractive-index layers and each of the laminated low-refractive-index layers are formed of the same metallic oxide. The refractive indices of the laminated layers are designed so as to increase in the order of their positions nearer to the transparent material. Thus, this technique achieves a low refractive index in the visible optical range.